As is known, night vision technology involves the collection of natural light in the lower portion of the infrared light spectrum and the artificial amplification of that natural light to such a point that images residing in the natural light can be readily observed. Based in part on Einstein's photoelectric theory, the modern Night Vision Device (NVD) enables a user to view objects, essentially in the dark. NVDs can amplify the light from the night sky by a factor of 30,000 times or more.
To effectively utilize a NVD, a user must subject the NVD to natural nighttime radiant intensity levels. These intensity levels may occur as a result of direct or indirect moonlight or starlight, or alternatively, by the deliberate production of artificial lighting that emulates the spectral irradiance of the night sky within the confines of an area that is normally not exposed to moonlight or starlight. It is known that the night sky exhibits a normal starlight spectral energy distribution in the 450 nanometer to 900 nanometer range of the electromagnetic spectrum. The spectral energy that resides from the 600 nanometer to 900 nanometer region is typically the area of sensitivity and amplification of the NVD.
Due to the advancements in image intensifier technology, the sensitivity and resolution of NVDs has improved greatly over the past few years. As a result, the role of NVDs, particularly within the military, has been expanding rapidly. For example, there exists a need for NVDs to work in concert with the artificial lighting that serves as the basis for backlighting liquid crystal display (LCD) generated graphics or for interior lighting within a crew station or a work area.
There are generally two classifications of artificial light emitting devices. The first classification is commonly referred to as a compatible spectral emission of an emitter. The compatible spectral emission of an emitter is defined as a non-NVD interacting spectral distribution of a light source wherein the spectral emission of the light source is outside the area of sensitivity of the NVD. Hence, the artificial light emitting device emits light that is apparent to the human eye, but not apparent to an observer using a NVD. This classification is generally referred to as Class A Night Vision Compatibility. The second classification is referred to as a tolerant spectral emission of a light source. The tolerant spectral emission of the light source is defined as a NVD interacting spectral distribution of the light source wherein a portion of the spectral emission of the light source is within the area of sensitivity of the NVD. Since the spectral distribution of the light source is within of sensitivity of the NVD, both the wearer of the NVD and a non-wearer of the NVD are able to distinguish the light source. This second classification is also generally referred to as Class B Night Vision Compatibility. It is noted, however, that the Class B NVD wearer can only detect levels of intensity of a tolerant light source and image contrast. Hence, the NVD wearer cannot detect the color of the light while observing the highly amplified scene.
The light sources that are considered compatible to both Class A and Class B NVDs are generally white light sources that have been converted through a series of optical filters to trim the normal spectral distribution of a light source to minimize the radiant energy in the 600 nanometer to 930 nanometer region of the electromagnetic spectrum. Class B NVD tolerant light sources are designed to interact with the NVD such that an observer utilizing the Class B NVD can safely view these sources of light as part of the amplified image of the NVD. The level of radiant energy emitted by these so-called friendly emitters do not interact in such a way as to activate the automatic gain control of the NVD and to interfere with the NVD's image resolution or performance. As is known, long-term, continuous exposure to high levels of NVD tolerant light levels will induce degradation to the image intensifier and shorten the life of the NVD. NVD friendly emitters can be fashioned into several colored emitters such as yellow for caution display functions and red for warning display functions.
The amount of allowable light energy that passes through the NVD to the observer can be quantified and is known as Night Vision Radiant Intensity (NRi) units. NRi units are defined as the integral of the curve generated by multiplying the spectral radiance or irradiance of the modified light source by the relative spectral response of the particular NVD used. It should be noted that excessive NRi units or light energy passing through the NVD may distort or alter the normal performance of the NVD and may cause the automatic gain control circuitry of some NVDs to activate, thereby rendering the device blind.
Most NVDs incorporate a phosphor screen to display the images to be observed by the wearer. It should be further noted that the amount of light energy allowed to transfer through the NVD relates directly to the overall brightness of the monochromatic fluorescent image produced by the phosphor screen. The level of the image brightness is important as not to cause nighttime blindness to an observer's eye after becoming dark-adapted. The luminance differences between the output of the phosphor screen and the un-amplified outside scene of the natural world must remain reasonably small to insure the best visual compatibility possible when compensating for the slower adaptation and luminous efficiency of the dark adapted eye, particularly if intense fluorescence was emanating from the phosphor screen.
Automatic gain control (AGC) is a method inclusive to some NVDs to allow the management of offending NRi where the luminous intensity level of the source of light energy is either too reflective or too emissive in nature. Since excessive light levels can cause permanent damage to the phosphor screen, management of these light levels by active AGC circuitry causes the image intensifier tubes to be reduced in their overall sensitivity to the incoming light resulting in a less intense image that is presented to the dark-adapted human eye. AGC will compensate for higher levels of ambient light, and in particular, to those levels of light energy in the area of sensitivity of the NVD. However, any light energy which activates the AGC will reduce the sensitivity of the NVD and cause poor image reproduction by the NVD. In extreme cases, the light energy may cause the AGC to shut down the NVD, thereby rendering the NVD blind to the outside world.
It should be noted that excessive NRi present within the amplified scene could induce an effect known as blooming. Blooming occurs when a NVD is exposed to a source of light that has not been corrected for excessive NRi. This source of light causes an intense brightness reproduction within the amplified scene of the NVD such that the resolution of the NVD is greatly reduced. This, in turn, destroys reproduction of the image of the outside scene by the NVD. If the NVD is being used in connection with the piloting air or watercraft, the hazards of blooming or temporary blindness are obvious.
The amount of NRi tolerated by these NVDs is based on the class and type of the particular NVD deployed and the kind of light emitter(s) selected. By way of example, the class and type of a NVD that is deployed for the use with a light emitter that produces a spectral emission outside the region of spectral sensitivity of the NVD (hereinafter referred to as a “non-NVD interacting light emitter”) would have an NRi value no greater than 1.0 E-10 as an order of magnitude when the emitter is providing a brightness level not exceeding 0.1 foot lamberts. It should be understood that a light emitter exhibiting this level of energy would not be detected or seen in the highly amplified scene produced by the NVD. For example, the non-NVD interacting light emitter may take the form of an advisory light, which is primarily observed by a non-NVD wearer and tends to be bluish green in color.
Similarly, the class and type of NVD deployed necessitate the use of NVD interacting light emitters that can be detected through the NVD without activating the AGC of this class and type of NVD. For example, a light emitter such as an emergency or warning light must be visible through the NVD for prompt action by a crewmember. In extreme applications of tolerant NRi, the level of NRi values can reach 1.0E-05 as an order of magnitude. It should be understood that tolerant NRi threshold values for caution and warning light emitters for Class B NVDs must at least generate 1.0E-09 to assure detectability through the Class B NVD. However, it is particularly advantageous to reduce the tolerant NRi level of a light emitter as much as possible where it is applicable.
In some applications, it is desirable to provide a large scale, area contained Night Vision Friendly (NVF) environment for enclosed compartments without windows. For the purposes of this discussion, a NVF environment is understood as the environment of the total interaction of confined light energy emitters whose spectral characteristics are well matched to perform a unified function as to artificially illuminate or cause to illuminate an immediate area to support vision amplification devices. It should be understood that light energy from an overhead source would interact with light energy transmitted from an electro-optical display particularly when both spectral emissions reach the human eye at the same time. Accordingly, the combined NRi levels of both source emitters must be compatible with the particular class and type of NVD deployed.
It should be recognized that the illumination of an interior compartment must provide adequate illumination to perform work. The definition of the illumination must include the appropriate brightness levels and subsequent chromatic characteristics to perform the work safely. However, more importantly, the illumination must not exhibit offensive NRi levels such that the light energy generated activates the AGC of the NVD when deployed within the enclosed compartment.
While the access doors of an enclosed compartment are usually closed, it can be appreciated that the doors must be occasionally opened for maintenance or the like. It can be appreciated that during the course of conducting night operations, the illumination characteristics of an NVF environment must not offend any NVD present if the compartment doors are breached. It should be further recognized that the control of the light energy signature that emanates from the various light emitters from military ships would be effective in reducing the propensity of detection by enemy NVD equipped personnel while at sea and mitigating the risk of piloting aircraft near or about the vicinity of the ship while aviators deploy NVDs.
To accomplish an NVF environment, optical filtering is usually required to suppress a substantial amount of light energy within the 600 nanometer to 930 nanometer range of the electromagnetic spectrum. This is also the portion of the electromagnetic spectrum to which the NVD is most sensitive. It can be appreciated that any spectral emitter converted to night vision compatibility (NVC) producing adequate energy throughout the visible electromagnetic spectrum would cause a predominate color shift in the spectral radiance of the artificial white light source.
It should be understood that a deliberate truncation of the visible spectrum (400 nm to 760 nm) of a light source to accommodate suppressing the region of NVD sensitivity beginning either at wavelengths 625 nm for Class A NVDs or 665 nm for Class B NVDs would result in the departure from the original or unfiltered polychromatic spectra of the light source. It should be further recognized that perceived changes in the color as well as the transreflective nature of the light source would be a function of the degree of suppression to the contributing wavelengths that compose the original polychromatic spectra. Hence, changes affecting the original spectral emission of the light source and associated chromatic expression of the source would be influenced either by suppression or elimination of the wavelengths that predominately influence the shade and color dominance of a white light source.
The tristimulus response of a light source and the sum of its corresponding ratios of blue, green and red spectral energies are generally the predominate factors that determine the degree of purity of the color generated by the light source. Any relatively unequal alteration of the source spectra by a filter will inherently induce changes in the perceived color of the light source and, of course, the illuminated scene. It should also be understood that trans-reflectivity characteristics of objects and light emitters would also be changed, notwithstanding the non-NVD wearer's ability to distinguish approximate object color rendering while illuminated, by truncated source irradiance. Further, any transmitted colors from electro-optical equipment would also provide the same difficulty to the observer.
If the departure from the original color spectra from the source were significant, then the ability for an observer to distinguish definitive reflective color in objects, specifically those whose spectral responses that lie beyond the truncated region, would be difficult. It is also apparent that subtle differences in transmitted color or colored images would also cause a non-NVD wearer to be less sensitive to subtle changes that occur during the presentation of displayed graphics or other types of annunciation by electro-optical means.
As a result, the efficiency of the lighting system and true display color generation after conversion would be compromised due to the suppression of light energy in the 600 nanometer to 760 nanometer region of the visible spectrum. Due to the absence of yellow, orange and red spectral components, the optical energy produced by the spectral emitters when transmitting light energy below 600 nanometers would be predominately bluish green in color. Hence, the successful suppression of this bandwidth results in the extremely low illumination intensity of a desired area by the light source, as well as, the generation of light having a blue to green color.
By way of example, New et. al., U.S. Pat. No. 6,515,413 discloses a method and apparatus that filters infrared light from fluorescent lighting and that is adapted to typical fluorescent lighting and assemblies. The apparatus includes a filter assembly comprising a transparent, cylindrical tube with a diameter and length slightly greater than those of a fluorescent tube. A cap may be placed on each end of the tube. Each cap is perforated to receive the electrical contacts of the fluorescent tube. The electrical contacts pass through the cap and can engage the electrical connections of a fluorescent fixture. Gaskets are placed between the caps and the ends of the fluorescent tube and prevent light from escaping through the perforations in the cap.
As described, the light source described in the '413 patent produces an output irradiance wherein the illumination intensity of the output irradiance is significantly less than is necessary for working environment and wherein the color of the illumination is green. The illumination level required to assure appropriate NVD usage is typically not greater than 5 foot candles. It can be appreciated that the lack of overall illumination intensity and the modification of the color of a light source may give rise to certain problems. More specifically, modification of the color of light produced by a light source may alter non-NVD wearers visual acuity and their ability to recognize colored objects, such as a crew member's ability to read colored maps/diagrams or to distinguish color coded wiring. As a result, safety issues may arise in certain situations where visual impairment by individuals tasked to perform work within the illuminated zone is to be expected. For example, crewmembers may be unable to properly recognize various warning and caution placards which are normally designed to reflect red, orange and yellow colored information while under normal white light illumination. However, due to the absence of sufficient yellow, orange and red spectral energy emanating from the modified light source will render red, orange and yellow colored objects or transreflective information unrecognizable.
Further, it can be appreciated that it is necessary to provide cooperative filtration on display equipment to further promote NVD compatibility within the NVF environment created by the NVF overhead lighting. In many military command and operations work/crew stations, many electro-optical displays utilize either Cathode Ray Tube (CRT) based technology or Liquid Crystal Display technology (LCD) to generate multi-colored graphics. However, the night vision radiant intensity (NRi) from any unfiltered local light emitter such as a CRT or LCD display within the NVF environment would defeat the NVF illumination environment and interfere with the NVD.
It is understood that there is a need to provide night vision compatibility (NVC) of a multi-color generating display and retain to some degree, enough spectral energy to reproduce yellow, orange and red spectral components to preserve non-NVD wearer's ability to see and distinguish colored information without compromising the NRi limits of the interfaced NVD or offend the NVF illumination environment. Furthermore, the condition of joint compatibility is also needed for every local light emitter present and active within the enclosed NVF illumination environment for any electro-optical device that generates a monochromatic or full color emission. It is further contemplated that additional safe guards of compatibility are required if the NVF environment was situated in an open compartment. It should be understood that an open compartment includes a workstation that requires the ability for a crewmember to view a scene of the outside world through a window or portal. Again, it is further contemplated that to provide the necessary security against detection from the outside world from enemy NVD and to provide NVC to friendly aviators or other crewmembers while deploying NVDs in close proximity to the compartments.
Therefore, it is a primary object and feature of the present invention to provide a method for generating night vision compatible illumination of an area.
It is a further object and feature of the present invention to provide a method for generating night vision compatible illumination of an area that does not cause a loss in resolution or sensitivity of the images observed by a wearer of a night vision device.
It is a still further object and feature of the present invention to provide a method for generating night vision compatible illumination of an area that allows an individual in the illuminated area without NVD to recognize reflected colors under all brightness operating conditions ranging from approximately 5 to 200 foot candles.
It is a still further object and feature of the present invention to provide a method for generating night vision compatible illumination of an area wherein the area is sufficiently illuminated for daytime usage, normal nighttime usage without NVDs and night operations with NVDs.
It is a still further object and feature of the present invention to provide a method for generating night vision compatible illumination of an area that does not interfere with ability of a wearer of a night vision device to recognize self-illuminating warning signals, or a non-NVD wearer from recognizing reflective colored placards, or reflective colored emergency exit signs.
It is a still further object and feature of the present invention to provide a method for generating night vision compatible illumination of an area that is fabricated from a material that is better than glass at resisting breakage.
In accordance with the present invention, a method is provided for suppressing a predetermined portion of an optical spectrum emanating from a light emitter. The method includes the step of fabricating a filter to absorb the predetermined portion of the optical spectrum. The filter has a spectrophotometric transmittance. Thereafter, the spectrophotometric transmittance of the filter is varied.
The step of fabricating the filter includes the step of forming a sheet of material having a predetermined cross sectional thickness. The sheet is fabricated from a polymer incorporating a series of colorants and infrared absorbing dyes. It should be understood that incorporating colorants within the sheet allows for minor chromatic correction for specific applications such as tinting or to create the influence of a saturated color if desired. It is contemplated that a multitude of variants could be derived from manipulation and modification of the basic colorant.
The step of varying spectrophotometric transmittance of the filter includes the step of removing a portion of the cross sectional thickness of the sheet. The sheet is partially defined by first and second surfaces. The step of removing a portion of the cross sectional thickness of the sheet includes the step of cutting the first surface of sheet in a circular pattern so as to define a modified surface. Thereafter, the modified surface may be restored.
It is contemplated for the predetermined portion of the optical spectrum absorbed by the filter to be in the range of 600 nanometers to 900 nanometers and to provide a curvature in the filter.
In accordance with a further aspect of the present invention, a method is provided for suppressing a predetermined portion of an optical spectrum emanating from a light emitter. The optical spectrum illuminates an illumination area. The method includes the steps of fabricating a filter having a spectrophotometric transmittance and varying the spectrophotometric transmittance of the filter. Thereafter, the filter is positioned between the light emitter and the illumination area. The filter absorbs the predetermined portion of the optical spectrum and transmits a predetermined portion of the visible spectrum.
The step of fabricating the filter includes the step of forming a sheet having a predetermined cross sectional thickness. The sheet is fabricated from a polymer incorporating a series of colorants and infrared absorbing dyes. The step of varying the spectrophotometric transmittance of the filter includes the step of removing a portion of the cross sectional thickness of the sheet. The sheet is partially defined by first and second parallel surfaces and the step of removing a portion of the cross sectional thickness of the sheet includes the step of cutting and removing the first surface of sheet in a circular pattern parallel to the second surface so as to provide a modified surface. The modified surface may be restored.
It is contemplated for the predetermined portion of the optical spectrum absorbed by the filter to be in the range of 600 nanometers to 900 nanometers and to provide a curvature in the filter.
In accordance with a still further aspect of the present invention, a method is provided for suppressing a predetermined portion of an optical spectrum emanating from a light emitter. The light emitter illuminates an illumination area. The method includes the step of fabricating a filter having a predetermined cross sectional thickness and a spectrophotometric transmittance. Thereafter, the cross sectional thickness of the filter is reduced. The filter is then placed between the illumination area and the light emitter.
The step of reducing the cross sectional thickness of the filter includes the step of cutting the filter in a circular pattern. The method may also include the additional step of positioning the filter between the light emitter and the illumination area. The filter absorbs the predetermined portion of the optical spectrum and transmits a predetermined portion of the visible spectrum.
It is contemplated for the predetermined portion of the optical spectrum absorbed by the filter to be in the range of 600 nanometers to 900 nanometers and to provide a curvature in the filter.